patch 9.0.0379: cleaning up after writefile() is a hassle

Problem:    Cleaning up after writefile() is a hassle.
Solution:   Add the 'D' flag to defer deleting the written file.  Very useful
            in tests.

src/vim9expr.c

@@ -833,6 +833,14 @@ compile_call(
 		}
 	    }
 
+	    if (STRCMP(name, "writefile") == 0 && argcount > 2)
+	    {
+		// May have the "D" flag, reserve a variable for a deferred
+		// function call.
+		if (get_defer_var_idx(cctx) == 0)
+		    idx = -1;
+	    }
+
 	    if (idx >= 0)
 		res = generate_BCALL(cctx, idx, argcount, argcount_init == 1);
 	}